There are many situations in which signals from sources at different locations need to be integrated or combined, e.g., a video signal and its associated audio signal. Another example is an video signal received from a satellite feed or microwave repeater relay network which may have its associated audio signal transmitted to the broadcast facility over land lines (e.g., telephone lines). In other cases a remote video feed may be integrated with a local video signal, e.g., in a news broadcast or a live interview between the broadcast facility and a remote location.
In general, video and audio or multimedia equipment uses an external “Genlock” reference signal for synchronization. Genlock is commonly used in video post-production, nonlinear editing, and broadcast studios, etc. Its use ensures that a video and audio or multimedia equipment's information output is locked (LOCK) to an externally generated signal (GEN), so that the studio's devices (e.g., cameras, videotape recorders, character or title generators, etc.) work together correctly. Through Genlock, the information output may be synchronized to an external source. The Genlock reference signal generally takes the form of a normal video/audio information signal, but without any program content.
A Genlock reference signal is produced by a sync generator that generates a signal of the same frequency as an information signal, but with precise timing signals that are not subject to frequency drift or time base errors which might affect the information signal. In an analog color video signal, for example, the sync generator generates horizontal sync, vertical sync, color reference, burst flag and blanking pulses, typically as a color black reference signal, at precise intervals corresponding to the nominal specifications for the format of the particular signal being processed.
While signals that conform to the NTSC standard or to the PAL standard are still in use today and will continue to be for the foreseeable future, the advent of digital technologies has enabled the widespread use of Standard Definition and High Definition television, both in the baseband and compressed (MPEG-2, -4, etc.) domains. The new digital TV standards utilize digital broadcasting/multimedia equipment that needs to be synchronized.
Video signals originating from and within a broadcast facility are Genlocked to the reference signal generated by a master generator, which provides a common timing reference for aligning the various signals. The phasing of two information signals of the video type involves multiple levels of alignment. First, the vertical alignment to the nearest vertical sync pulse is performed. Then, the correct horizontal line is aligned. On the next level, each picture is synchronized spatially (horizontal and vertical). In an analog composite system, additional phasing of the subcarrier is finally carried out to achieve the correct color reproduction.
In modern digital systems, the concept of subcarrier no longer exists and only horizontal and vertical phasing is required. Two signals which exhibit vertical and horizontal (and subcarrier, if applicable) alignment are said to by synchronous, and can be used together without further timing adjustment in a production process such as switching, mixing, superimposition, etc.
In some situations it may become necessary to combine information signals originating from sources at different locations. The lengths of the signal paths may differ, such as where a video signal is relayed via satellite and its associated audio signal is transmitted over land lines, which causes a delay differential between the signals, or independent video signals containing information relating to a common event. For example, for a local interviewer interviewing a person at remote location, or a panel discussion between panelists in different cities, may arrive at the broadcast facility at different times if the signal paths differ. Each remote signal source may be locked to an independent reference signal, which may not be in precise alignment with other unrelated reference signals or the reference signal used by the broadcast facility.
The Society of Motion Picture and Television Engineers (SMPTE) and the European Broadcasting Union (EBU) are looking into developments in the broadcast infrastructure synchronization (see, e.g. www.ebu-acip.org). Audio over IP (Internet Protocol) terminals are increasingly being used in radio operations for streaming of radio programs over IP networks from remote sites or local offices into main studio centers. The Internet is increasingly also used for various scenarios of video/audio/multimedia contribution, especially over longer distances. News correspondents will have the choice in their equipment to use either ISDN or the Internet via ADSL or other available IP networks to deliver their reports. With very few exceptions, IP equipment from one manufacturer has until now not been compatible, let alone synchronizable, with another manufacturer's unit.
One approach to signal synchronization is outlined in U.S. Pat. No. 6,191,821 (Kupnicki). Another approach is provided in FR 2007/050918, which is directed to transmitting a synchronizing signal in a packet network having two or more stations. Image cues are extracted from a synchronizing signal. A first counter is initialized based on the image cues. A program dock reference counter is initialized every second or higher zero crossing of the first counter. The program clock reference counter samples all the Tech periods. Tech is derived from a time base synchronized on all the network stations. Packets containing the samples are transmitted in the network. The synchronizing signal is a Genlock signal. The time base synchronized on all the network stations is an IEC 1588 time base.
Another example is FR 2007/050919, which is directed to a secure mechanism for transmission of a synchronizing signal in a packet network. A device transmits packets in a packet communication network comprising two or more stations. The device has a signal receiver and can control a counter using the received signal. The counter samples every Tech periods. Tech is derived from a time base synchronized on all the stations of the network. An incremental value predicting at least a next sampled value of the counter is calculated. Sampled values and associated incremental values are transmitted in packets. The time base synchronized on all the network stations is an IEC 1588 time base. The received signal is of Genlock (synchronization lock) signal. The communication network is an IP (Internet Protocol) network.
Still another example is FR 2007/050914, which is directed to generating and operating a double temporal descriptor for transmitting a synchronizing signal in a packet network. Packets are transmitted in a packet communication network comprising two or more stations. A signal receiver is adapted to derive a frequency and a phase from a received signal. A master counter is provided and initialized by the derived frequency and phase. A sampling device samples the value of the master counter at regular intervals. From a sampled value of the master counter, a first temporal descriptor is generated. A second counter is synchronized on all network stations at the same instant when the sampling of the master counter occurs. A second temporal descriptor is generated from a sampled value of the second counter. The two temporal descriptors are jointly transmitted in the network. The packet communication network is an IP network. The second counter is synchronized on all the network stations by means of the IEC 1588 standard. The received signal is a Genlock signal.